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SubjectRe: [PATCH RFC tip/core/rcu 4/5] srcu: Provide polling interfaces for Tiny SRCU grace periods
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Hi Paul,

On 11/17/2020 6:10 AM, paulmck@kernel.org wrote:
> From: "Paul E. McKenney" <paulmck@kernel.org>
>
> There is a need for a polling interface for SRCU grace
> periods, so this commit supplies get_state_synchronize_srcu(),
> start_poll_synchronize_srcu(), and poll_state_synchronize_srcu() for this
> purpose. The first can be used if future grace periods are inevitable
> (perhaps due to a later call_srcu() invocation), the second if future
> grace periods might not otherwise happen, and the third to check if a
> grace period has elapsed since the corresponding call to either of the
> first two.
>
> As with get_state_synchronize_rcu() and cond_synchronize_rcu(),
> the return value from either get_state_synchronize_srcu() or
> start_poll_synchronize_srcu() must be passed in to a later call to
> poll_state_synchronize_srcu().
>
> Link: https://lore.kernel.org/rcu/20201112201547.GF3365678@moria.home.lan/
> Reported-by: Kent Overstreet <kent.overstreet@gmail.com>
> [ paulmck: Add EXPORT_SYMBOL_GPL() per kernel test robot feedback. ]
> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> ---
> include/linux/rcupdate.h | 2 ++
> include/linux/srcu.h | 3 +++
> include/linux/srcutiny.h | 1 +
> kernel/rcu/srcutiny.c | 52 ++++++++++++++++++++++++++++++++++++++++++++++--
> 4 files changed, 56 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index de08264..e09c0d8 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -33,6 +33,8 @@
> #define ULONG_CMP_GE(a, b) (ULONG_MAX / 2 >= (a) - (b))
> #define ULONG_CMP_LT(a, b) (ULONG_MAX / 2 < (a) - (b))
> #define ulong2long(a) (*(long *)(&(a)))
> +#define USHORT_CMP_GE(a, b) (USHRT_MAX / 2 >= (unsigned short)((a) - (b)))
> +#define USHORT_CMP_LT(a, b) (USHRT_MAX / 2 < (unsigned short)((a) - (b)))
>
> /* Exported common interfaces */
> void call_rcu(struct rcu_head *head, rcu_callback_t func);
> diff --git a/include/linux/srcu.h b/include/linux/srcu.h
> index e432cc9..a0895bb 100644
> --- a/include/linux/srcu.h
> +++ b/include/linux/srcu.h
> @@ -60,6 +60,9 @@ void cleanup_srcu_struct(struct srcu_struct *ssp);
> int __srcu_read_lock(struct srcu_struct *ssp) __acquires(ssp);
> void __srcu_read_unlock(struct srcu_struct *ssp, int idx) __releases(ssp);
> void synchronize_srcu(struct srcu_struct *ssp);
> +unsigned long get_state_synchronize_srcu(struct srcu_struct *ssp);
> +unsigned long start_poll_synchronize_srcu(struct srcu_struct *ssp);
> +bool poll_state_synchronize_srcu(struct srcu_struct *ssp, unsigned long cookie);
>
> #ifdef CONFIG_DEBUG_LOCK_ALLOC
>
> diff --git a/include/linux/srcutiny.h b/include/linux/srcutiny.h
> index fed4a2d..e9bd6fb 100644
> --- a/include/linux/srcutiny.h
> +++ b/include/linux/srcutiny.h
> @@ -16,6 +16,7 @@
> struct srcu_struct {
> short srcu_lock_nesting[2]; /* srcu_read_lock() nesting depth. */
> unsigned short srcu_idx; /* Current reader array element in bit 0x2. */
> + unsigned short srcu_idx_max; /* Furthest future srcu_idx request. */
> u8 srcu_gp_running; /* GP workqueue running? */
> u8 srcu_gp_waiting; /* GP waiting for readers? */
> struct swait_queue_head srcu_wq;
> diff --git a/kernel/rcu/srcutiny.c b/kernel/rcu/srcutiny.c
> index 3bac1db..b405811 100644
> --- a/kernel/rcu/srcutiny.c
> +++ b/kernel/rcu/srcutiny.c
> @@ -34,6 +34,7 @@ static int init_srcu_struct_fields(struct srcu_struct *ssp)
> ssp->srcu_gp_running = false;
> ssp->srcu_gp_waiting = false;
> ssp->srcu_idx = 0;
> + ssp->srcu_idx_max = 0;
> INIT_WORK(&ssp->srcu_work, srcu_drive_gp);
> INIT_LIST_HEAD(&ssp->srcu_work.entry);
> return 0;
> @@ -114,7 +115,7 @@ void srcu_drive_gp(struct work_struct *wp)
> struct srcu_struct *ssp;
>
> ssp = container_of(wp, struct srcu_struct, srcu_work);
> - if (ssp->srcu_gp_running || !READ_ONCE(ssp->srcu_cb_head))
> + if (ssp->srcu_gp_running || USHORT_CMP_GE(ssp->srcu_idx, READ_ONCE(ssp->srcu_idx_max)))
> return; /* Already running or nothing to do. */
>
> /* Remove recently arrived callbacks and wait for readers. */
> @@ -147,14 +148,19 @@ void srcu_drive_gp(struct work_struct *wp)
> * straighten that out.
> */
> WRITE_ONCE(ssp->srcu_gp_running, false);
> - if (READ_ONCE(ssp->srcu_cb_head))
> + if (USHORT_CMP_GE(ssp->srcu_idx, READ_ONCE(ssp->srcu_idx_max)))

Should this be USHORT_CMP_LT ?

> schedule_work(&ssp->srcu_work);
> }
> EXPORT_SYMBOL_GPL(srcu_drive_gp);
>
> static void srcu_gp_start_if_needed(struct srcu_struct *ssp)
> {
> + unsigned short cookie;
> +
> if (!READ_ONCE(ssp->srcu_gp_running)) {
> + cookie = get_state_synchronize_srcu(ssp);
> + if (USHORT_CMP_LT(READ_ONCE(ssp->srcu_idx_max), cookie))
> + WRITE_ONCE(ssp->srcu_idx_max, cookie);

I was thinking of a case which might break with this.

Consider a scenario, where GP is in progress and kworker is right
before below point, after executing callbacks:

void srcu_drive_gp(struct work_struct *wp) {
<snip>

while (lh) {
<cb execution loop>
}
>>> CURRENT EXECUTION POINT

WRITE_ONCE(ssp->srcu_gp_running, false);

if (USHORT_CMP_LT(ssp->srcu_idx, READ_ONCE(ssp->srcu_idx_max)))
schedule_work(&ssp->srcu_work);
}

Now, at this instance, srcu_gp_start_if_needed() runs and samples
srcu_gp_running and returns, without updating srcu_idx_max

static void srcu_gp_start_if_needed(struct srcu_struct *ssp)
{
if (!READ_ONCE(ssp->srcu_gp_running)) returns true
<snip>
}

kworker running srcu_drive_gp() resumes and returns without queueing a
new schedule_work(&ssp->srcu_work); for new GP?

Prior to this patch, call_srcu() enqueues a cb before entering
srcu_gp_start_if_needed(), and srcu_drive_gp() observes this
queuing, and schedule a work for the new GP, for this scenario.


WRITE_ONCE(ssp->srcu_gp_running, false);
- if (READ_ONCE(ssp->srcu_cb_head))
+ if (USHORT_CMP_GE(ssp->srcu_idx, READ_ONCE(ssp->srcu_idx_max)))
schedule_work(&ssp->srcu_work);

So, should the "cookie" calculation and "srcu_idx_max" setting be moved
outside of ssp->srcu_gp_running check and maybe return the same cookie
to caller and use that as the returned cookie from
start_poll_synchronize_srcu() ?

srcu_gp_start_if_needed()
cookie = get_state_synchronize_srcu(ssp);
if (USHORT_CMP_LT(READ_ONCE(ssp->srcu_idx_max), cookie))
WRITE_ONCE(ssp->srcu_idx_max, cookie);
if (!READ_ONCE(ssp->srcu_gp_running)) {
<snip>
}


Thanks
Neeraj

> if (likely(srcu_init_done))
> schedule_work(&ssp->srcu_work);
> else if (list_empty(&ssp->srcu_work.entry))
> @@ -196,6 +202,48 @@ void synchronize_srcu(struct srcu_struct *ssp)
> }
> EXPORT_SYMBOL_GPL(synchronize_srcu);
>
> +/*
> + * get_state_synchronize_srcu - Provide an end-of-grace-period cookie
> + */
> +unsigned long get_state_synchronize_srcu(struct srcu_struct *ssp)
> +{
> + unsigned long ret;
> +
> + barrier();
> + ret = (READ_ONCE(ssp->srcu_idx) + 3) & ~0x1;
> + barrier();
> + return ret & USHRT_MAX;
> +}
> +EXPORT_SYMBOL_GPL(get_state_synchronize_srcu);
> +
> +/*
> + * start_poll_synchronize_srcu - Provide cookie and start grace period
> + *
> + * The difference between this and get_state_synchronize_srcu() is that
> + * this function ensures that the poll_state_synchronize_srcu() will
> + * eventually return the value true.
> + */
> +unsigned long start_poll_synchronize_srcu(struct srcu_struct *ssp)
> +{
> + unsigned long ret = get_state_synchronize_srcu(ssp);
> +
> + srcu_gp_start_if_needed(ssp);
> + return ret;
> +}
> +EXPORT_SYMBOL_GPL(start_poll_synchronize_srcu);
> +
> +/*
> + * poll_state_synchronize_srcu - Has cookie's grace period ended?
> + */
> +bool poll_state_synchronize_srcu(struct srcu_struct *ssp, unsigned long cookie)
> +{
> + bool ret = USHORT_CMP_GE(READ_ONCE(ssp->srcu_idx), cookie);
> +
> + barrier();
> + return ret;
> +}
> +EXPORT_SYMBOL_GPL(poll_state_synchronize_srcu);
> +
> /* Lockdep diagnostics. */
> void __init rcu_scheduler_starting(void)
> {
>

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